I think that you could definitely rig one to fit in your window by pinching the exhaust vent in the casement window. The tricky part would be closing off the rest of the window to ensure that you don't lose all of that cold air back to the outside.
If you're handy and don't mind losing all of the light from that window for the summer, you could always do kind of what you said in your first post - take the window and screen out and make a big piece of wood/plastic to fill the window and then put a hole to match the size of the exhaust vent.
If I made it, it would look hideous. But I would rather be cool anyway. Your level of handiness might make it look presentable.
Edit: And I have never seen anything that would fit a casement window perfectly, unfortunately. Some amount of improvisation would definitely be needed, whether it involved the entire window frame, or just the open part.